1. Field of the Invention
This invention relates to a preparation of a printed wiring board utilizing an electrophotographic reverse developing method which uses a material to be developed having a metal conductive layer and a photoconductive layer in this order on at least one surface of an insulating substrate, particularly to a liquid developing method which gives a resist image by applying a liquid toner developing treatment onto the material to be developed having a static latent image.
2. Prior Art
A method for making a printed wiring board is roughly classified into two methods of the subtractive method and the additive method. The subtractive method is a method in which a resist layer is formed on a laminated board to which a conductive layer such as copper, etc. is provided on an insulating substrate, and the conductive layer not covered by the resist layer is removed by etching. The additive method is a method in which a conductive layer is formed only at a wiring pattern portion on an insulating substrate. In addition to the above, it has been proposed a wiring transfer method in which a resist image is provided on a conductive substrate by a plate resist, metal plating is applied to the conductive substrate other than the resist image to form a metal wiring pattern, and then, after the resist image is removed, only the metal wiring pattern is transferred to the insulating material.
Preparation of a resist layer utilizing an electrophotographic method is carried out as mentioned below. A photoconductive layer is provided on a copper-clad laminated board, and after uniformly charging the surface of the photoconductive layer in a dark place, exposure is carried out according to a wiring pattern whereby charging at the exposed portion disappears and an electrostatic latent image can be formed. The electrostatic latent image is subjected to toner developing treatment and fixing to form a toner image, and by using the toner image as a resist, the photoconductive layer other than the toner image portion is dissolved and removed to prepare a resist image of a metal conductive layer comprising the toner image and the photoconductive layer. Dissolution and removal of the unnecessary portion of the metal conductive layer and the subsequent preparation step of the printed wiring board can be carried out in the same manner as in the conventional procedure.
When the electrophotographic method by the reverse developing method (hereinafter simply referred to as xe2x80x9can electrophotographic methodxe2x80x9d) is used, procedure is carried out that the surface of a photoconductive layer is charged in a dark place and forming an electrostatic latent image is formed by vanishing a charge at the exposed portion by exposing an image portion, and then, fine toner particles having the same polarity as those of the charged electric charge are adhered to the exposed portion, i.e., to a portion at which the electric charge is vanished. At this time, a conductive member called a developing electrode is provided opposed to the surface of an electrostatic latent image, and a bias voltage with the same polarity is applied to the electrode whereby adhesion of the toner particles to the exposed portion is promoted and a toner image with a high image quality can be obtained.
A method of preparing a resist layer to the both surfaces of a substrate utilizing the electrophotographic method has been disclosed in Japanese Provisional Patent Publication No. 224541/1994, and it is possible to form toner images on the both surfaces by simultaneously developing both surfaces of a laminated board in which a metal conductive layer and a photoconductive layer are provided on the both surfaces of an insulating substrate.
At present, in a progress of high density preparation of a printed wiring board, a printed wiring board having a lot of conductive fine pores called through holes provided to respective substrate with a multi-layered structure has been prepared. In this case, circuits formed on the both surfaces of the substrate are connected by the through holes. Thus, not only precision of the circuit itself formed on the substrate but also precision of formed positions of the circuits on the both surfaces relative to the through holes as a standard, i.e., positional precision prior to formation of the resist film are required.
Moreover, according to the recent trend of making light, thin, short and compact, or variety of electronic devices, in a printed wiring board, it is now required to make it with high density and within a short period of time. Also, a direct drawing method by the use of data from a computer using a flat bed scanning exposure device which uses laser beam or light emitting diode in place of the conventional surface exposure drawing method using a photomask has now been investigated. An application of an electrophotographic material has been investigated as a resist material.
As a flat bed scanning exposure device to be used for preparing a resist film utilizing the electrophotographic method, it is possible to use a device which uses various kinds of lasers or light emitting diode as a light source. During exposure, it is carried out by moving at least either one of an optical system or a material to be exposed placed on a flat bed.
When a both-surfaces printed wiring board is to be prepared by using the flat bed scanning exposure device to be used in the above-mentioned direct drawing method, there may be mentioned a method in which one surface of a laminated board is statically charged and exposed, then the laminated board is turned over, and the other surface of the laminated board is statically charged and exposed to effect both surfaces exposure, and toner development is carried out on the both surfaces simultaneously, or a method in which one surface of a laminated board is statically charged, exposed and subjected to toner development, and then, the other surface of the same is similarly treated to form a resist pattern. In the former method, an electrostatic latent image is contacted with the flat bed after turn over so that the electrostatic latent image is likely disturbed whereby any measure to prevent such a disturbance is required. To the contrary, in the latter method, a non-developed surface is not statically charged at the time of toner development so that there is a possibility of causing stain at the non-developed surface due to an excess amount of the toner. However, the latter method is capable of processing the surface to which an electrostatic latent image is formed without contacting any material. Thus, the latter method (hereinafter referred to as xe2x80x9ca one-surface stepwise developing systemxe2x80x9d) has been investigated nowadays as a process for producing a both-surfaces printed wiring board by the direct drawing method.
Moreover, when a both-surfaces printed wiring board having a through hole(s) is to be prepared, depending on the size of a diameter of the through hole(s) and a thickness of a material to be developed, a problem occurs that toner is electrodeposited on a non-developed surface side of a penetrated through hole(s) by a bias voltage to cause a defect at the time of liquid toner development by carrying out the above-mentioned one-surface stepwise developing system.
Also, in the above-mentioned one-surface stepwise developing system, when a thin copper-clad laminated board having a thickness of 0.4 mm or less is used as a substrate, the substrate bends between electrodes (which also varies depending on the size of the material to be developed) so that the change in the distance of gaps between the electrode and the substrate causes a problem. At this time, a bottom side of the substrate is contacted with rolls to move the substrate so that a non-developed surface statically charged is contacted with the rolls whereby the statically charged state of said non-developed surface is disturbed. Also, a liquid toner migrated from the development surface side to the non-development surface side adheres to said non-development surface ununiformly so that stain slightly remains at the non-development surface side in some cases. Thus, the method finally brings a result that it is inferior in reliability as a preparation process of a printed wiring board.
In the above-mentioned phenomenon, stain at the back surface can be prevented when the non-development surface is not statically charged contrary to the above. In this case, however, an electric field of the development surface migrates to around the hole of a relatively large-sized through hole(s) or a register hole provided at an edge portion of the printed wiring board whereby the liquid toner is electrodeposited at around the hole(s) which causes a problem of stain.
Also, the above conveying rolls of the electrode portion is subjected to contact conveying so that stain caused by the liquid toner developer is accumulated on the roll surface whereby stain on the surface of the non-development material occurs in some cases.
On the other hand, with regard to these liquid developing methods, it is essential requirement that an excessive liquid toner shall not be brought into the substrate after the developing treatment by the liquid toner, and to realize the requirement, various kinds of processing devices or processing methods have been proposed. For example, in a printed wiring board processing device disclosed in Japanese Provisional Patent Publication No. 102582/1996, by providing at least one pair of squeezing rolls at the downstream side of developing electrodes and at least one pair of liquid-absorbing rolls having capillary action at the further downstream side of the squeezing rolls, an excessive amount of liquid toner can be completely removed including an excessive solution in a through hole(s) even when the material to be developed has a through hole(s). In this reference, as a constitution of the pair of the liquid squeezing rolls, two liquid-absorbing rolls are generally employed.
However, when a large number of sheets are intermittently carried out for a long period of time by the liquid development using the above-mentioned method, a balance between a toner particle amount and a charge thereof of the liquid toner possessed by the liquid absorbing roll becomes bad due to the reason of condensation, etc. At this time, the material to be developed is conveyed while absorbing the liquid toner at the state that a nip is loaded under a constant pressure to the pair of liquid-absorption rolls. Simultaneously, the liquid toner possessed by the pair of the liquid-absorption rolls is exhausted at the upper stream side so that if a charge balance of the liquid toner exhausted is bad, a formed pattern is sometimes disturbed in some cases.
An object of the present invention is, when preparing a both-surfaces printed wiring board by the electrophotographic reverse developing method, to provide a liquid developing method which can prevent occurrence of an image defect caused by liquid toner stain at a non-developing surface when the one-surface stepwise developing system is applied irrespective of a thickness of a both-surfaces printed wiring board, which can prevent occurrence of a liquid toner stain at the non-developing surface including a neighbor of a through hole(s) when a both-surfaces printed wiring board having a through hole(s) is prepared, which can remove an excessive amount of the toner simply, easily and efficiently in the through hole(s) and the surface of an electrophotographic laminated board after the developing processing and can realize to form a wiring pattern corresponding to an electrostatic latent image for a long period of time with good reproducibility.
The present inventors have earnestly studied to accomplish the above objects, and as a result, they have found that these objects can be accomplished by a liquid developing method of a printed wiring board which comprises, for preparing a printed wiring board by applying a one-surface stepwise developing system which forms a resist pattern on each surface stepwisely according to an electrophotographic reverse developing method using a material to be developed obtained by forming photo-conductive layers on both surfaces of a both-surfaces copper-clad laminated board, subjecting a non-electrostatic latent image formed surface which is an opposite surface to a latent image formed surface which is a surface to be exposed, i.e., a non-developing surface to static charging treatment during completion of an exposure treatment and before a liquid toner developing treatment, and then, subjecting the non-developing surface to the liquid toner developing treatment whereby a toner image corresponding to the resist pattern can be formed on the electrostatic latent image surface.
That is, in either of the surfaces, by subjecting a non-electrostatic latent image formed surface to which no developing treatment is desired to be carried out to static charging treatment, a toner image can be formed on an electrostatic latent image-formed surface (a surface to be developed) by the developing treatment, and also, unexpected formation of a toner image on a non-developed surface which is a non-electrostatic latent image-formed surface can be prevented. By applying a role of the operation as mentioned above to the case where a toner image is to be formed by subjecting to the liquid toner developing treatment in place of the above-mentioned non-developed surface, a high quality toner image without no toner stain on the both surfaces of the material to be developed can be formed.
For producing a printed wiring board to which resist patterns are formed on the both surfaces of a material to be developed obtained by formation of photoconductive layers on the both surfaces of a both-surfaces copper-clad laminated board by applying the one surface stepwise developing system to the board, the procedure can be carried out as follows: First, a static charging treatment is applied to the non-developing surface during the time period of after completion of the exposure treatment and before the liquid toner developing treatment, then, a toner image corresponding to a resist pattern is formed on an electrostatic latent image-formed surface by the liquid toner developing treatment. Thereafter, the material to be developed is overturned, an electrostatic latent image is formed on the non-developed surface in the same manner as mentioned above. Then, the static charging treatment is applied to the surface opposite to the electrostatic latent image-formed surface (i.e., the toner image formed surface) and the liquid toner developing treatment is applied thereto to form a resist pattern on the other surface of the material to be developed.
Moreover, when a printed wiring board is prepared by applying a one-surface stepwise developing system which forms a resist pattern on each surface stepwisely according to an electrophotographic reverse developing method using a material to be developed having a through hole(s), which is obtained by forming photoconductive layers on both surfaces of a both-surfaces copper-clad laminated board, it is possible to prevent occurrence of toner stain at around a through hole(s) at a non-developed surface by subjecting to the same or similar treatment as mentioned above.
A surface potential of the non-developing surface obtained as a result of the statically charging treatment to the above-mentioned non-developing surface may be 80 V or more.
Also, the liquid developing method of a printed wiring board according to the above-mentioned method can be accomplished by, in a liquid developing device which is subjecting to a liquid toner developing treatment of a material to be developed on which an electrostatic latent image is provided by applying a suitable static charging on one surface of a material to be developed obtained by forming photoconductive layers on the both surfaces of a both-surfaces copper-clad laminated board and to an exposure treatment, a liquid developing device of a printed wiring board which comprises a liquid toner feeding portion, and a statically charging means for statically charging the surface of said material to be developed substantially immediately before a developing electrode portion. However, said statically charging means is not necessarily provided in said liquid developing device and it may be provided at substantially immediately after the exposure treatment portion to substantially immediately before the portion at which the liquid toner is fed to the electrostatic latent image surface.
Accordingly, the liquid developing method of the present invention is a method of obtaining resist patterns on the both surfaces of a material to be developed, which comprises forming an electrostatic latent image on one surface of the material to be developed according to a statically charging step and an exposure step, and then, applying statically charging, liquid toner development, drying and fixing treatments on a non-developed surface to form an electrostatic latent image on the back surface of the material to be developed again, and subjecting the back surface to the same treatment as in the front surface.
Moreover, the present inventors found that the above-mentioned objects can be accomplished by a liquid developing method of a printed wiring board which comprises, among the steps of preparing a printed wiring board by applying a one-surface stepwise developing system which forms a resist pattern on each surface stepwisely according to an electrophotographic reverse developing method using a material to be developed obtained by forming photoconductive layers on both surfaces of a both-surfaces copper-clad laminated board, when liquid development is carried out by transferring the material to be developed with a state that a bottom side of the material to be developed as a non-developing surface is substantially level with the ground, substantially level transferring of the material to be developed at developing electrodes portion is carried out with electrodes portion conveying rolls and simultaneously a liquid toner may be fed to the whole surface of said non-developing surface substantially uniformly.
They have also found that by closely contacting a liquid-absorption sheet to the surfaces of said electrodes portion conveying rolls from the bottom portion of said electrodes portion conveying rolls, no problem occurs even when a large number of sheets are treated intermittently for a long period of time. More specifically, when the liquid-absorption sheet is adhered to the surface of the electrodes portion conveying rolls during they are rotating, it is sufficient if the state that the roll surfaces of the electrodes portion conveying rolls which contact with the material to be developed are not stained can be maintained.
They have further found that these objects can be accomplished by a liquid developing method of a printed wiring board which comprises, for preparing a printed wiring board by applying a one-surface stepwise developing system which forms a resist pattern on each surface stepwisely according to an electrophotographic reverse developing method using a material to be developed obtained by forming photoconductive layers on both surfaces of a both-surfaces copper-clad laminated board, removing an excessive amount of a toner at the liquid toner developing treatment by combination of a liquid-absorption rollhaving a capillary function at the non-developing surface side and a roll made of an elastic material having no liquid-absorbing property at the developing surface with regard to a pair of liquid-squeezing rolls provided at the downstream side of the developing electrodes.